As the requirement for the volume-related power density (VA/in3) of a switched-mode power supply increases, the requirements for its inductive components, in particular the main transformer or transformers, also increase. Therefore, for about 20 years, printed circuit card transformers of every conceivable design have increasingly being used, as a separate component or integrated into the main board of a power supply.
One example of such a printed circuit card transformer is known from U.S. Pat. No. 5,010,314. Its primary and secondary coils are etched-in on printed circuit cards, which have a recess in their center, so that the printed circuit cards can be fitted one on top of the other onto the ferrite core of the transformer, with an insulating layer being provided between neighboring printed circuit cards. The printed circuit cards are held together by a coil former comprising two halves, the printed circuit card being disposed with the primary coil between the two halves and the secondary windings being arranged on the mutually averted sides of the halves of the coil former. All the printed circuit cards are embraced by legs which run around on both sides of the halves of the coil former. The ferrite core comprises two E-shaped halves, the coil former carrying the printed circuit cards being fitted onto the middle leg of one of the halves of the ferrite core and the other half of the ferrite core being fitted on from the other side of the coil former.
This type of printed circuit card technology is used primarily for signal transformers, storage inductors and transformers in the power range up to about 150 VA.
In the power range above 150 VA, with outputs with small voltages (<12 V) and correspondingly high output currents, considerable quality problems arise in the manufacture of printed circuit card transformers. For instance, in the case of high currents, the copper thickness of the printed circuit cards must be correspondingly great and then no longer conforms to the standard of the printed circuit card industry.
In the case of high output powers, comparatively expensive printed circuit cards with special thicknesses are required; it may be necessary for standard copper thicknesses to be built up with copper. If printed circuit cards with special copper thicknesses are used, the etching gap between the interconnects can only be guaranteed with optimum process setting. Even the smallest deviations in the process or contaminations cause tiny copper bridges between the interconnects. Such a bridge between two interconnects results in an inadequate number of turns, an interturn short-circuit or, with a conducting connection between the interconnect and the outer edge, even safety-relevant creepage paths between the windings or between the winding and the ferrite core. Such a conducting connection between two interconnects can only be detected during printed circuit card manufacture by elaborate measuring methods directly after the respective process step, or it is only detected in the final functional testing of the completely assembled transformer. However, the value added is lost and much of the material used can no longer be put to further use.
Alternatively, a number of thin layers of copper of multilayered printed circuit cards can be connected in parallel. However, the total thickness of such a printed circuit card is comparatively high because of the insulating layers between the conductor layers. There is also the disadvantage that exact connection of the parallel conductor layers in the printed circuit card is laborious and is only possible with covered vias if required safety standards are to be met.
A further problem, specifically in the case of upright printed circuit card transformers, is the mechanically stable and current-resistant contacting of the printed circuit card with all the required inner layers to the printed circuit board, for example a main board of the power supply.